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Tekkaya, Ahmet Eeman
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Name Variants
Tekkaya, A. E.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
Tekkaya, A. Erman
Tekkaya, AE
A.E.Tekkaya
Tekkaya,A.E.
T.,Ahmet Eeman
A.,Tekkaya
A., Tekkaya
T., Ahmet Eeman
Tekkaya, Ahmet Eeman
Ahmet Eeman, Tekkaya
Tekkaya, E
Tekkaya, E. A.
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Profesör Doktor
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Scholarly Output
33
Articles
15
Citation Count
867
Supervised Theses
2
33 results
Scholarly Output Search Results
Now showing 1 - 10 of 33
Article Citation Count: 14Finite deformation plasticity coupled with isotropic damage: Formulation in principal axes and applications(Elsevier Science Bv, 2010) Soyarslan, Celal; Tekkaya, Ahmet Eeman; Manufacturing Engineering; Department of Mechatronics EngineeringA local, isotropic damage coupled hyperelastic-plastic framework is formulated in principal axes. It is shown that, in a functional setting, treatment of many damage growth models, including those originated from phenomenological models (with formal thermodynamical derivations), micromechanics or fracture criteria, proposed in the literature, is possible. As a model problem, a Lemaitre-variant damage model with quasi-unilateral damage evolutionary forms is given with special emphasis on the feasibility of formulations in principal axes. To this end, closed form expression for the inelastic tangent moduli, consistent with the linearization of the closest point projection algorithm, is derived. It is shown that, generally, even in the absence of quasi-unilateral damage evolutionary conditions, the consistent tangent moduli are unsymmetric. The model is implemented as a user defined material subroutine (UMAT) for ABAQUS/Standard. The predictive capability of the selected model problem is studied through axi-symmetric application problems involving forward extrusion of a cylindrical billet, upsetting of a tapered specimen and tension of a notched specimen, in which characteristic failure mechanisms are observed. (C) 2010 Elsevier B.V. All rights reserved.Article Citation Count: 50Residual stress state and hardness depth in electric discharge machining: De-ionized water as dielectric liquid(Taylor & Francis inc, 2005) Tekkaya, Ahmet Eeman; Elkoca, O; Erden, Abdulkadir; Erden, A; Manufacturing Engineering; Department of Mechatronics EngineeringProcedures and results of experimental work to measure residual stresses and hardness depth in electric discharge machined surfaces are presented. Layer removal method is used to express the residual stress profile as a function of depth caused by a die sinking type EDM. Thin stressed layers are removed from machined samples by electrochemical machining. Corresponding deformations due to stress relaxation are recorded for each removal to determine the stress profile from elasticity theory. The relational dependence of the machining parameters with residual stresses is obtained and a semi-empirical model is proposed for plastic mold steel for de-ionized water as dielectric liquid. These stresses are found to be increasing rapidly with respect to depth, attaining to its maximum value, around the yield strength, and then fall rapidly to compressive residual stresses in the core of the material since the stresses within plastically deformed layers are equilibrated with elastic stresses.Article Citation Count: 9Free forming of locally heated specimens(Elsevier Sci Ltd, 2007) Tekkaya, Ahmet Eeman; Ozmen, M.; Huwiler, H.; Tekkaya, A. E.; Manufacturing EngineeringA novel manufacturing method is investigated, in which a steep temperature gradient within the workpiece is induced to facilitate material flow locally. By this method, complex shapes can be formed without complicated dies. The feasibility of the idea is analyzed experimentally and numerically. Local heating is realized either by means of induction or laser heating. Experiments using materials 16MnCr5, X5CrNi18/9, and Ti6Al4V have been conducted under various process conditions. These experiments have also been modeled by finite element method (FEM) validating the analysis procedure. Electromagnetic models are used to analyze the heat generation pattern on the workpiece by induction. It is found that the most important process parameters are the thermal diffusivity and the temperature sensitivity of the flow curve of the workpiece material. The lower the thermal diffusivity and the higher the temperature sensitivity, the more differentiated local shapes can be formed. For the analyzed geometries, induction heating has been observed to be more effective. Deformation rate and initial workpiece geometry have also a significant effect on the achievable local deformations. Various failure modes such as unintended deformations, damage by fracture, and melting of the workpiece material are described. It is concluded that the new idea of forming local shapes by local heating is a feasible and controllable manufacturing alternative. (c) 2006 Elsevier Ltd. All rights reserved.Article Citation Count: 7Generalized transient temperature behavior in induction heated workpieces(Elsevier Science Sa, 2009) Tekkaya, Ahmet Eeman; Dursunkaya, Zafer; Tekkaya, A. Erman; Manufacturing EngineeringElectromagnetic-thermal coupled numerical analysis of induction heating is time intensive if three-dimensional models are used. In this study, by processing the results of finite element computations, a dimensional analysis is carried out to predict the transient temperature rise time at the surface of an induction heated workpiece. A unique set of curves is found that describes the transient dimensionless temperature at a point on the surface as a function of the electromagnetic skin depth alone. The effectiveness of the approach is verified by numerical calculations and comparing the dimensionless temperature curves for different heating cases and materials. Effect of temperature dependent thermal properties is also investigated. The consistency of numerical results is also verified by comparison with experiments. (C) 2009 Elsevier B.V. All rights reserved.Conference Object Citation Count: 127Incremental bulk metal forming(Elsevier, 2007) Tekkaya, Ahmet Eeman; Fritsche, D.; Tekkaya, E. A.; Allwood, J. M.; Hirt, G.; Neugebauer, R.; Manufacturing EngineeringIncremental bulk forming is the oldest known technique in metal working. Many developments in this field have dramatically changed our society. Today incremental bulk forming processes are applied to quality products in small and large series production. Numerous advances have been realized. The motivation for using these processes is presented here. After a general definition of incremental processes and a classification of incremental bulk forming in particular, some innovative product and process examples are given that show the potential. Finally recent progress and challenges are illustrated in detail. This includes the development of new machinery for incremental bulk forming, advanced methods for process planning, occurrence of failure modes and the properties of finished products.Conference Object Citation Count: 2Influence of Heat Treatment and Grinding Conditions on Surface Residual Stresses in the Production of Rollers(Trans Tech Publications Ltd, 2006) Tekkaya, Ahmet Eeman; Güley,V.; Tekkaya,A.E.; Savaş,T.; Özhan,F.; Manufacturing EngineeringThe aim of this study is to investigate surface residual stresses after heat treatment and grinding processes in the production of rollers. The residual stresses were measured using the X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on AISI-E52100 (100Cr6) ball bearing steel. Taguchi design of experiments (DOE) is applied to define the set of experiments for grinding, which facilitates evaluation of the individual influences of process parameters on residual stresses and also eliminates unnecessary experiments. Response of residual stresses to each parameter is evaluated with the help of the results of residual stress measurements by X-ray diffraction. In grinding with aluminum oxide wheels, it was concluded that the lower the cutting speed and the higher the workpiece speed the higher the magnitude of surface compressive residual stresses. Higher compressive stresses were measured in axial direction compared to the circumferential direction after the grinding process.Conference Object Citation Count: 2Comparison of the Deep Drawability of Aluminum and Steel Using Numerical Simulation Experiments(American Institute of Physics Inc., 2005) Sönmez,C.; Tekkaya, Ahmet Eeman; Tekkaya,A.E.; Gür,C.H.; Manufacturing EngineeringSheet metal forming processes, especially deep drawing processes give diverse results by various materials. Extreme differences occur between steel sheets and aluminum sheets. The main causes of these differences are variances in micro- and macroscopic material properties, such as anisotropy. In this study, the behavior of two distinct materials, steel and aluminum alloy, during an axisymmetrical cup drawing operation has been studied numerically. For this purpose, finite element (FE) simulations of a simple cup drawing process, which was studied in the benchmarks of the NUMISHEET 2002 have been conducted using a commercial dynamic-explicit FE-analysis package. The materials analyzed have been 6111-T4 aluminum alloy and mild steel graded as deep drawing quality. Basic process parameters, which are the blank holding force and the lubrication condition, have been varied to obtain a "successful" product and the process windows for these two materials have been compared and investigated. Thickness distributions in the blank, force requirements for the process and product quality have been used for the basis of comparison. The results are also compared with an analytical model developed by Ramaekers. © 2005 American Institute of Physics.Article Citation Count: 6Material Flow Control in High Pressure Sheet Metal Forming of Large Area Parts With Complex Geometry Details(verlag Stahleisen Mbh, 2005) Trompeter, M; Tekkaya, Ahmet Eeman; Önder, E; Homberg, W; Tekkaya, E; Kleiner, M; Manufacturing EngineeringWorking media based forming processes show advantages compared to the conventional deep drawing in the range of sheet metal parts with complex geometry details. By High Pressure Sheet Metal Forming (HBU), complex parts can be formed with reduced tool costs, fewer process steps, and improved part properties, particularly by the use of high strength steels. In order to use these advantages to full capacity, the material flow into the area of the geometry details needs to be optimised. The key element for the material flow control is a multi-point blank holder. In combination with flange draw-in sensors, a closed loop flange draw-in control can be built up which guarantees a reproducible material flow and, consequently, defined part properties. Furthermore, a favourable pre-distribution of sheet metal material can be reached which leads to a widening of the process limits. Considering a large area sheet metal part with a complex door handle element as example, strategies for the material flow control will be discussed in this paper. The conclusions are based on FE-simulations as well as experimental findings.Conference Object Citation Count: 3Experimental Investigation of Residual Stresses After Heat Treatment and Grinding Processes in the Production of Ball Bearing Rings(Trans Tech Publications Ltd, 2008) Güley,V.; Tekkaya, Ahmet Eeman; Tekkaya,A.E.; Savaş,T.; Özhan,F.; Manufacturing EngineeringExperimental investigation of residual stresses after heat treatment and grinding processes in the production of ball bearing rings has been carried out. The residual stresses were measured by X-ray diffraction method utilizing chromium radiation, which has an average penetration depth of 5 μm incident on 100Cr6 (AISI-E52100) ball bearing steel. The process parameters of heat treatment and grinding processes were varied so as to represent the extreme values that can be applied in the respective processes. Hardness and percent retained austenite limit the heat treatment process parameters; while roundness, surface roughness and form the grinding process. Tensile surface residual stresses on the raceway of ball bearing rings changes to compression after grinding in both circumferential and axial directions. In grinding relatively higher compressive stresses were measured in axial direction compared to the circumferential direction. This experimental investigation also showed that the influence of heat treatment process parameters on the magnitude and distribution of residual stresses survived even after grinding process; i.e. heat treatment and grinding processes cannot be evaluated independently in process design for favourable residual stresses.Article Citation Count: 38Application of Continuum Damage Mechanics in Discontinuous Crack Formation: Forward Extrusion Chevron(Wiley-v C H verlag Gmbh, 2008) Soyarslan, Celal; Soyarslan, Celal; Tekkaya, A. Erman; Akyuz, Ugurhan; Tekkaya, Ahmet Eeman; Department of Mechatronics Engineering; Manufacturing EngineeringMaterializing Continuum Damage Mechanics (CDM), numerical modeling of discrete internal cracks, namely central bursts, in direct forward extrusion process is presented. Accordingly, in a thermodynamically consistent setting, a local Lemaitre variant damage model with quasi-unilateral evolution is coupled with hyperelastic-plasticity. The formulations are constructed in the principal axes where simultaneous local integration schemes are efficiently developed. To this end, the framework is implemented as ABAQUS/VUMAT subroutine to be used in an explicit FE solution scheme, and utilized in direct forward extrusion simulations for bearing steel, 100Cr6. Discontinuous cracks are obtained with the element deletion procedure, where the elements reaching the critical damage value are removed from the mesh. The periodicity of the cracks shows well accordance with the experimental facts. The investigations reveal that, application of the quasi-unilateral conditions together with the crack closure parameter has an indispensable effect on the damage accumulation zones by determining their internal or superficial character. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
